Typically, in a telephone network a hybrid converter connects the unidirectional four-wire link from the public switched telephone network (PSTN) to the local two-wire loop. The hybrid converter separates the transmitted signal originating in the local loop from the received signal in the four-wire link from the PSTN, and the transmitted signal in the PSTN from the received signal in the local loop. Since there is an impedance mismatch in the hybrid converter, part of the received energy of a signal is reflected back to the transmitting port, causing a speaker to hear his own delayed speech, i.e., an electrical echo. A second type of echo is caused when a speech signal generated from a speaker at a terminal propagates in the form of an acoustic wave through an acoustic environment and part of it is captured by the microphone of the terminal. When the acoustic wave is transmitted back to the speaker, it is called an acoustic echo. To suppress echoes, typically a suppressor switch monitors the voice signals travelling in both directions. The switch detects which person is talking and blocks the signal traveling in the opposite direction. Such suppressor switches may terminate speech signals too quickly when the speakers speak fast, thus causing some speech information to be lost when the suppressor switch does not switch fast enough. Of course, when speakers speak simultaneously, the suppressor switch is ineffective. When communication is implemented in a multicarrier-based modulation system having Digital Subscriber Loops (DSLs), data bits need to be allocated to form Quadrature Amplitude Modulation (QAM) consistent with the American National Standards Institute (ANSI) while optimizing data transmission capacity within the constraints of the limited power budget and power mask constraint specified by ANSI. ANSI is an organization that coordinates, develops, and publishes standards for use in the United States.
While the ANSI standard for Asynchronous Digital Subscriber Loops (ADSL) permits Discrete Multitone (DMT) modulation in full-duplex transmission mode over a limited bandwidth, most modems do not use full-duplex transmission. Apparently, no scheme has been designed to spectrally allocate data bits under the above conditions and the literature does not include descriptions of any such efforts. The ANSI standard permits duplex transmission, that is, transmitting and receiving data over the same bandwidth over the same pair of wires, which is possible by the use of an analog hybrid circuit that isolates the transmitted signal from the incoming received signal. The hybrid can yield isolation of 15-20 dB. In other words, the transmitted signal that leaks into the received signal is attenuated by about 15-20 dB from its transmitted power level. The received data, however, has been attenuated by the channel between the remote transmitter and the receiver. Therefore, the signal transmitted by the remote transmitter can be attenuated to a much higher degree as it reaches the remote receiver. For a 3,000 ft. wire, the attenuation increases to −20 dB over the 1 MHz bandwidth that is used by an asymmetric digital subscriber loop (ADSL) modem. For a 18,000 ft. wire, the attenuation can be as much as −140 dB. Thus, the received signal is generally of much lower power than the leakage signal. The contamination by the leakage signal must be reduced by the implementation of an Echo Canceler (EC). Since the EC cannot completely cancel the leakage, the residual leakage acts as receiver noise and reduces the signal-to-noise ratio (SNR) of the received signal. This leakage signal is generally not measured, and it adds to the noise power that is measured during the initialization period of the modem.
All multicarrier DSL modems require a scheme to allocate data bits among the frequency bins (subcarriers) of each complex transmission symbol. Any such scheme uses the channel attenuation characteristics and the measured noise background as inputs and provides a distribution of data bits across frequency bins as the output. Some schemes described in the prior art use a variation of an approach often referred to as the ‘water filling’ approach. These schemes consider the limitation of total power as the primary constraint within which the bit allocation must be determined. The constraints of minimum and maximum number of bits per bin and power spectral mask are not considered. Other schemes in the prior art can be used to determine the bit allocation subject to all such practical constraints. However, none of the prior art schemes perform optimal allocation if full-duplex transmission is desired.
Thus, there is a need for a symmetric DSL modem that efficiently allocates data bits to subcarriers for full-duplex transmission mode to overcome the limitations cited above.